Although the immediate effort leading to this disclosure is directed toward the lumbar spine (anterior or posterior in approach), the described vertebral implants for immediate fixation and staged stabilization leading to arthrodesis (bone fusion) of bone bodies may be used in a bone fracture or osteotomy to fuse together resulting bone bodies, and across one or more joints or articulations. Furthermore, the implants may be used in the lumbar, thoracic and cervical spine.
To facilitate fusion and healing of fractured bones, it has long been known to utilize fixation plates and screws to hold together disunited bone bodies. Typically, the separate bone bodies are formed when a single bone fractures, requiring bone reunion. Plates are secured across a fracture region with screws, joining together the bone bodies. The plates hold the bone bodies together in proximate relation, facilitating bone growth and fusion there between. In this manner, the bone bodies are supported in close proximity, or in direct contact which facilitates fusion there between. For cases where it is impossible to fixture together bone bodies internally of a patient's skin, external fixation is used. For external fixation, threaded pins are rigidly secured into each bone body. The pins, which extend outwardly of a patient's skin, are fixtured together with an external fixation device, placing the bone bodies in adjacent proximate position to promote healing there between. However, this is not practical for certain joints such as joints formed between spinal vertebrae.
An early technique for achieving arthrodesis between adjacent bone bodies across a joint or articulation is the well known Cloward Technique for use in the human cervical spine. A solitary dowel of bone is tapped into place in a prepared circular bed that is smaller than the dowel of bone. The dowel acts as a wedge, distracting the surrounding soft tissues of the joint, and separating the bone bodies or vertebrae joined there along. The intervertebral disc substantially comprises the soft tissues of the joint. The dowel of bone is inserted, or wedged into place, providing its own stability by putting an annulus of the disc on stretch. Additionally, simple friction of the inserted dowel between adjacent vertebral bodies stabilizes axial dislocation. However, a second surgical procedure must be performed to extract or harvest the dowel of bone, substantially adding trauma to the procedure, increasing costs, as well as increasing the threat of infection to the patient. Alteratively, bank bone from human donors can be used, but bank bone is less osteogenic and may introduce infection, or even transmission of Acquired Immune Deficiency Syndrome (AIDS) or hepatitis. Furthermore, bone morphogenic protein, hydroxyapatite, or other bone stimulating material may be utilized. Additionally, there has been a need to ensure the implant remains axially secured which has lead to further developments.
As a step forward from the Cloward Technique, the Bagby metal dowel (U.S. Pat. No. 4,501,269) utilizes the same principle. A perforated cylindrical hollow implant is inserted between prepared surfaces across a vertebral joint. The inserted implant immediately stabilizes the joint by spreading the bony surfaces apart in wedged opposition to surrounding tissue. This initial stabilization is more substantial because a metal dowel, unlike a bone dowel, will not be absorbed or fatigue in use. Over time, fusion occurs through and around the implant which is filled with bone fragments. Use of the metal dowel eliminates the need for a second operation to harvest a dowel of bone. Bone fragments to be inserted in the implant are retrieved during preparation of the circular beds in each vertebra. Furthermore, such a metal implant avoids the disadvantage of having to use bone banks to obtain donor bone. The Bagby implant described in U.S. Pat. No. 4,501,269 has a smooth outer surface, interrupted only by numerous openings or fenestrations through which bone ingrowth and through growth can occur. Bone morsels or bone grafts are typically harvested when preparing the circular bed in each vertebra, after which they are placed into the fenestrated metal cylindrical implant. The Bagby implant is then driven or tapped into place in a manner similar to the placement of Cloward's Bone Dowel, which was solely directed for use in the cervical spine.
Improvements have also been made to "Cloward's Technique" wherein two dowel bone grafts are posteriorly inserted (Wiltberger's Technique) between adjacent lumbar vertebral bodies. Furthermore, threaded surfaces have been added to such bone grafts in order to keep the grafts in place (Otero-Vich German Application Number 3,505,567, published Jun. 5, 1986). More recently, a number of U.S. Patents have proposed combining the threaded features from threaded bone grafts with a metal implant, resulting in rigid threaded implant structures for placement between adjacent spinal vertebrae.
One threaded metal fusion implant disclosed in Michelson (U.S. Pat. No. 5,015,247) provides a cylindrical fusion implant having an outer diameter sized larger than the space between adjacent vertebrae to be fused. Threads provided on the exterior of the member engage the vertebrae to axially secure the implant there between. The implant has a plurality of openings configured along the cylindrical surface to promote bone ingrowth. However, the threads per se of the implant do not function as a fastener to fix together the adjacent vertebral bodies. Instead, the implant functions as a wedge, imparting a distraction force across the disc which stabilizes the articulation formed there between by stretching the annulus of the disc. In fact, the threaded implant relies solely on the annulus to provide stabilization between the vertebrae, in direct response to wedge-induced distraction created there between. Distraction of the annulus stabilizes the two vertebrae, enabling ingrowth to later occur within the implant. Therefore, through-growth and fusion (arthrodesis) occur between the adjacent vertebrae subsequent thereto depending on the immobilizing potential of an intact healthy annulus which may or may not be present. Therefore, there is a need to provide an implant that produces immediate fixation per se between bone bodies following insertion and independent of the annulus. Particularly for cases where the annulus structure is substantially or completely weakened or damaged at surgery of implantation, the wedge-effect of prior art threaded implants will not produce any distraction forces across the annulus. Also, when the implant is used to arthrodese and change angulation, a healthy annulus cannot be totally corralled to be placed on stretch. As a result, there is no form of stabilization or fastening between bone bodies sufficient to enable the occurrence of arthrodesis there between when the annulus is weakened or inadequate.
Another threaded implant disclosed in Ray (U.S. Pat. No. 5,005,104) provides a threaded fusion cage that is configured to be implanted in close adjoining pairs between adjacent vertebral bodies. Threads of adjacent cages are configured in overlapping relation when they are implanted. However, the fusion cages function only as wedges, imparting distraction forces across the annulus. The distraction forces immediately stabilize the intervertebral articulation by stretching the annulus of the disc immediately after implantation. Over time, the adjacent vertebrae fuse together. However, where a stretched annulus does not provide sufficient stabilization, initial early bone growth is seriously hindered, if not completely prevented. Furthermore, a stretched annulus can still allow slight motion.
For bone fusion to occur with any of the above devices, the invasion of new delicate blood vessels from the adjacent healthy bone is necessary for the creation of new living interconnecting bone. Where complete stabilization does not occur instantaneously upon implantation, motion can disrupt the ingrowth of delicate blood vessels. Disruption of the vessels then restricts or even prevents bone healing there between. The same problem occurs with any of the above mentioned implant techniques, including the threaded techniques of Otero-Vich and Michelson. Even when the annulus is completely on stretch, the threads per se of these constructions do not function in the manner of conventional screws, extending through one object and into another. Namely, they do not function to fasten together adjacent bodies by coaction of the thread with each body. Alternatively, they do not fasten together bodies by action of the thread with one body, and action of a fastener head with the other body. Instead, the threads merely act as a series of ridges that engage with each adjacent bone body, while the implant body functions as a wedge. The implant distracts apart the vertebral bodies which stretches the annulus, and stabilizes the articulation as a consequence thereof, while the thread functions solely to prevent axial dislodgement.
A further area of prior art relates to implants having surface features that enable bony ingrowth to occur. For example, beads of titanium have been provided on the stems of hip implants to form such features. Ingrowth by a bone bed with the structural features occurs some time after implantation. Therefore, fixation is not immediately present as a result of the surface features, and some other fixation must be relied upon until ingrowth occurs. With the exception of the Cloward Bone Dowel and Otero-Vich, the above-mentioned vertebral body implant devices incorporate fenestrations or openings that tend to facilitate bony ingrowth into the metal spinal implants.
Additionally, Lin et al. (U.S. Pat. No. 4,778,469) teaches a surface construction of a space occupier having a pattern for tissue ingrowth in the surface of an implant. Tapered posts having undercuts are provided along a surface of a hip implant. Subsequent to implantation, physiological bone ingrowth occurs within the undercuts, helping to fix the implant within the bone. However, this construction does not enable immediate fixation via the undercuts. Instead, it relies upon physiological bone ingrowth which takes time to occur. Therefore, other mechanisms must be relied upon to maintain implant fixation within the bone prior to ingrowth. Such is also the case with the previously mentioned vertebral implants.
Therefore, there is a present need to provide implant devices that fasten bone bodies together directly upon implantation. There is also a need to provide such a device that facilitates staged stabilization, ultimately leading to bone fusion there between. The final stage of bone fusion through and around the implant substantially eliminates any need for the implant to maintain the fusion, thus allowing the bone union to provide primary support there between, i.e. the implant can be removed without reversing the arthrodesis in such cases as chronic infection. Furthermore, there is a need to provide such a device for fixing bone bodies together across an articulation or joint (arthrodesis). Particularly, this need exists where soft tissues of an articulation have deteriorated to such a condition that distraction across the articulation will not produce stability. For example, prior art devices (including the above-mentioned vertebral body implant devices) cannot stabilize an articulation by inducing a wedging apart, or stretching of an annulus where the annulus is weakened or absent. Therefore, interim stability cannot be imparted between adjacent vertebrae at the time of surgery. Such interim stability is needed for successful fusion. As a result, ingrowth and through growth needed to fuse the bone bodies together for long-term stability is less likely to occur on a routine basis.